Broadly, the invention relates to an apparatus and method for evaluating thermal hazard characteristics of reactive chemicals. More specifically, this concept is directed to a dynamic adiabatic calorimeter for measuring the self-heating rate of exothermic chemical reactions, and which is used to obtain data such as thermal runaway potential.
The term calorimetry can be generally defined as measurement of energy in the form of heat. The science of calorimetry is widely used in the chemical industry to measure the quantity of heat liberated or absorbed during chemical reactions, changes of state, formation of solutions, and the like. Heat measurement data from chemical reactions can be of value for several reasons. For example, when chemical engineers design new plants or processes, it is essential to know the heat of reaction of the chemicals involved in order to properly calculate heat balances.
Over a period of many years several different types of calorimeters have been developed for measuring heat energy. One type of calorimeter in common use is an instrument referred to as an adiabatic reaction calorimeter. In the operation of an adiabatic calorimeter, the objective is to minimize heat transfer between the vessel (bomb) which contains the reacting sample and the structure and atmosphere which surrounds the vessel. If the adiabatic condition can be successfully maintained during the chemical reaction, such a system provides an excellent means for obtaining accurate heat measurement data.
It is generally known that chemical compositions are constantly undergoing decomposition. If the decomposition reaction is exothermic, heat energy is continuously evolved until all reactants are consumed. A general rule is that the amount of heat liberated will be proportional to the rate of the reaction. In some situations, however, because of low heat conduction to the environment, the heat being liberated during reaction will accumulate in the reacting mass faster than it can be carried away. For example, this situation will frequently occur when a chemical composition begins reacting in a confined space such as a reactor or a tank car.
To explain further, when the liberated heat causes the temperature of the mass to increase, it causes the reaction to go much faster, or accelerate, as the temperature climbs. Not only does the rise in temperature cause the reaction to go faster, it actually increases the rate of reaction in an exponential manner. For example, the reaction rate for many chemical compositions will double for each 10 degree rise in temperature, at ambient temperature. At a point where the reaction mass begins to generate more heat than the system is capable of removing, the reaction will undergo a thermal runaway.
The great release of energy from a thermal runaway can oftentimes cause vaporization of the chemical. In addition, the reaction of the chemical any evolve gaseous products. These factors may result in pressuring of the vessel in which the chemical is contained, and if the strength of the vessel is inadequate, it will rupture. The resulting explosion can cause costly damage to the process or storage area and may even result in serious personal injury.
Because of the thermal runaway hazard involved in the manufacture, storage and shipping of reactive chemicals, there is need for an instrument which can predict the runaway potential, or other adverse behavior of the chemical compositions, from temperature measurement data. Several of the commercially available instruments, including some adiabatic calorimeters, are useful for determining overall reaction information, such as heat of reaction, heat of combustion, heat of formation, and the like. Some instruments, such as differential thermal analysis instruments and differential scanning calorimeters, also scan the reactive chemical relative to an inert reference sample, to determine qualitatively whether the chemical exotherms or endotherms and the relative size of the reactions. To date, however, there is no known system capable of quantitatively determining the thermal runaway characteristics of reactive chemicals.
In contrast to the prior instruments, the present invention is an accelerating rate calorimeter, which provides a unique instrument in the field of calorimetry. In this instrument we are able to simulate, on a small scale, an actual thermal runaway of a reactive chemical. Not only can a thermal runaway be simulated, but the instrument does not require a reference sample to detect the exothermic reaction. Because the data is generated in an adiabatic environment, it can be applied directly to an actual situation.
Some of the prior calorimeter instruments operate only on a temperature base, that is, they are designed to determine overall energy information by observing only the adiabatic temperature rise in exothermic reactions. By contrast, the instrument of this invention obtains data based on temperature rise as a function of time. The instrument thus generates quantitative information on the adiabatic rate of reaction in the form of self-heat rate throughout the entire temperature range of the experiment. From this data the adiabatic (non-isothermal) kinetics which describe the self-accelerating reaction can be determined.
A particularly unique feature of this instrument is the capability to relate the experimental time to maximum rate, or time of explosion, as a function of temperature. This feature enables determining the thermal runaway potential of the reactive chemical at any temperature in the range of the experiment. Also, by extrapolation, this potential can be determined for any temperature below the expermental range. This hazard information can be applied directly to establish safe conditions for manufacturing, storing and shipping of reactive chemicals.